I bought the E-520 last week to replace my damaged E-510. So far I don't see too much difference between the two. According to what I've read the main difference between the two cameras is the improvements to the Live View. It adds a new contrast based auto focus system to the Live View and sports a larger LCD. Otherwise, the camera seems identical to the E-510. I've read that the dynamic range and image quality is improved with the sensor on the E-520 but it's too early for me to tell.
As far as the feel of the camera goes, it is very similar. While I think the weight difference between the E-510 and E-520 is minimal, I can feel the difference, or at least I think I do.
I have not had a lot of time to shoot with the camera yet, but I did bring it to the baseball game yesterday along with the 70-300mm lens. This is not the easiest lens to use nor is it the best quality glass I own so it's hard to make a judgement. So far, it seems like the images are bit over-saturated. I'd say the images also seem a bit soft but again that is probably my difficulty in using the lens at the long end.
Here's a shot of Derek Jeter hitting a home run, tying him with Lou Gehrig for the all time hit record at Yankee Stadium:
Going a bit longer the image seems to get a little soft:
Taking it all the way out to 300mm:
I won't really have a good feel for the camera until I spend some time with the 50mm macro and the kit lenses, as I know the quality of the Macro is very good and the kit lenses are much easier to handle.